Evidence that monochloramine disinfectant could lead to elevated Pb levels in drinking water.

Many water districts have recently shifted from free chlorine (in the form of HOCl/OCl-) to monochloramine (NH2-Cl) as a disinfectant for drinking water to lower the concentration of chlorinated hydrocarbon byproducts in the water. There is concern that the use of NH2Cl disinfectant may lead to higher Pb levels in drinking water. In this study, the electrochemical quartz crystal microbalance is used to compare the effects of these two disinfectants on the dissolution of Pb films. A 0.5 microm thick Pb film nearly completely dissolves in a NH2Cl solution, but it is passivated in a HOCl/OCl- solution. X-ray diffraction analysis shows that the NH2Cl oxidizes Pb to Pb(II) species such as Pb3-(OH)2(CO3)2, whereas the stronger oxidant, HOCl/OCl-, oxidizes Pb to Pb(IV) as an insoluble PbO2 conversion coating. Although NH2Cl may produce less halogenated organic byproducts than HOCl/OCl- when used as a disinfectant, it may lead to increased Pb levels in drinking water.